leds: ledtrig-heartbeat: Make top brightness adjustable

LED class heartbeat trigger allowed only for blinking with max_brightness
value. This patch adds more flexibility by exploiting part of LED core
software blink infrastructure.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>